pnm: Decode PAM format variants which have alpha

[mstoeckl]

This implements the transparent tupltypes listed in the PAM spec, at https://netpbm.sourceforge.net/doc/pam.html.

• I am submitting this PR to a) complete PAM format support so that image viewers based on image can read all
  reasonably constructable PAM images b) ensure that the image crate itself can read all the PAM tuple types that it
  writes.

• The format technically allows non-RGBA tuple types and depths > 4, but these should be rejected as they
  have no clear interpretation as an image.

• I've only added simple tests in-file for PAM, extending the existing ones, but if desired I can make regression test images.

• Other than by using image itself, PAM images with alpha channels can be produced by NetPBM and ImageMagick.
  For example:

pngtopam -alphapam rgb_alpha.png > pam_rgb_alpha.pam
pngtopam -alphapam gray_alpha.png > pam_grayscale_alpha.pam
convert rgb_alpha.png pam_rgb_alpha.pam
convert gray_alpha.png pam_grayscale_alpha.pam
convert blackandwhite_alpha.png -depth 1 pam_blackandwhite_alpha.pam

(Note: pngtopam does not produce BLACKANDWHITE_ALPHA; pamthreshold can, but isn't general and appears to have bugs; so ImageMagick's convert is the only standard converter which works. On the other hand, going from PAM to PNG, ImageMagick's convert incorrectly flips the alpha channel, while pamtopng works correctly.)

• I wasn't able to find detailed documentation on the ColorType memory layout, but assume, based on how the other encoders and decoders handle alpha, that pixel types like ColorType::Rgba16 are stored unpremultiplied, with native-endian channels in R,G,B,A memory order, and a linear alpha channel.